Method for making shapeable microcellular poly lactic acid articles

ABSTRACT

A method for making a shapeable article from poly(lactic acid) includes treating solid poly(lactic acid) that results in the solid poly(lactic acid) having a crystallinity of at least 20% by weight based on the weight of the solid poly(lactic acid) and a gas concentration of 6% to 16% by weight based on the weight of the solid poly(lactic acid); and heating the solid poly(lactic acid) having said minimum crystallinity and gas concentration to produce a cellular poly(lactic acid) article that is shapeable. The shapeable cellular poly(lactic acid) article is advantageous in that the article can be further shaped by heat and/or pressure (or vacuum), such as via thermoforming, into a variety of useful products.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.12/673161, filed Aug. 4, 2010, which is the national stage ofInternational Patent Application No. PCT/US2008/073360, filed Aug. 15,2008, which claims the benefit of U.S. Provisional Application No.60/956,092, filed Aug. 15, 2007, the disclosures of all of which areincorporated herein expressly by reference.

BACKGROUND

A solid-state foaming process is illustrated in FIG. 1 wherein foamingoccurs while the polymer remains in the solid state throughout thefoaming process. This process differs from other standard polymerfoaming processes because the polymer is not required to be in a moltenstate. Generally, at the beginning of the method, the polymer is inequilibrium with the surrounding temperature and pressure so that thepolymer is “unsaturated.” In block 102, the thermoplastic polymer istreated at an elevated pressure to cause the thermoplastic polymer toabsorb gas. The treatment of the polymer in block 102 may be carried outin a pressure vessel, which is sealed, and then the material is exposedto a high pressure inert gas such as, but not limited to, carbon dioxidewithin the pressure vessel. The high pressure gas will then start todiffuse into the thermoplastic polymer over time, filling the polymer'sfree intermolecular volume. The gas will continue to saturate thepolymer until equilibrium is reached. In block 104, the fully saturatedpolymer is removed from the saturation pressure to an environment oflower pressure so that the polymer is thermodynamically unstable,meaning that the polymer is supersaturated with gas and is no longer atequilibrium with the surrounding environment. The polymer will start todesorb gas from its surface into the surrounding environment. Desorptionof some gas is desirable in some circumstances, for example, to avoidthe creation of cellular structure in some areas of the polymer, such asat the surfaces. Desorption of the polymer can occur when thehigh-pressure gas is vented from the pressure vessel or the saturatedthermoplastic polymer is removed into ambient atmospheric pressure.Heating of the partially saturated polymer in block 106 is carried outat a temperature below the melting temperature of the neat polymer.Heating produces a cellular thermoplastic polymer. Since the polymer isstill in a solid state, the foams thus produced are called solid-statefoams to distinguish them from foams that are produced in an extruderfrom a polymer melt. The cellular thermoplastic polymer is less densethan the noncellular polymer, thus saving material costs. However,depending on the polymer, the size of the cells, and relative density,the cellular polymer may or may not possess desirable characteristics.

In the last few years, “bio-based” solid poly(lactic acid) or “PLA” hasbeen produced in large quantities for use in food and beverage packagingapplications. This has led to a rise in the production of extruded solidPLA sheet or rod for thermoforming or molding food packaging andprotective packaging that will biodegrade. Solid PLA articles use morematerials and are heavier as compared to foams or cellular materials.Accordingly, it would be desirable to produce articles made fromcellular PLA that can be subsequently thermoformed or molded similar tosolid PLA.

SUMMARY

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

A first embodiment relates to a method for making a shapeable articlefrom poly(lactic acid), includes treating solid poly(lactic acid) thatresults in the solid poly(lactic acid) having a crystallinity of atleast 20% by weight based on the weight of the solid poly(lactic acid)and a gas concentration of 6% to 16% by weight based on the weight ofthe solid poly(lactic acid); and heating the solid poly(lactic acid)having said minimum crystallinity and gas concentration to produce acellular poly(lactic acid) article that is shapeable.

A second embodiment relates to the method of the first embodiment,comprising treating the solid poly(lactic acid) with one or more gasesat a pressure in the range of 3 MPa to 5 MPa.

A third embodiment relates to the method of the first embodiment,comprising treating the solid poly(lactic acid) with one or more gasesat a pressure in the range of 2.75 MPa to 7 MPa.

A fourth embodiment relates to the method of the first, second, or thirdembodiment, wherein the one or more gases comprise carbon dioxide.

A fifth embodiment relates to the method of the first embodiment,further comprising treating the solid poly(lactic acid) with one or moregases at a pressure in the range of 3 MPa to 5 MPa to allow the one ormore gases to be absorbed followed by treating the solid poly(lacticacid) at atmospheric pressure at a temperature in the range of −20° C.to 25° C. to allow the one or more gases to desorb from the solidpoly(lactic acid).

A sixth embodiment relates to the method of the first through fifthembodiments, further comprising heating the solid poly(lactic acid)having said minimum crystallinity and said range of gas concentration ata temperature in the range of 40° C. to 100° C. to produce the cellularpoly(lactic acid) that is shapeable.

A seventh embodiment relates to the method of the first embodiment,wherein treating comprises a period of gas saturation followed by aperiod of gas desorption to provide a minimum crystallinity of 20% byweight and the gas concentration of 6% to 16% by weight.

An eighth embodiment relates to the method of the seventh embodiment,wherein the solid poly(lactic acid) is completely saturated before theperiod of gas desorption.

A ninth embodiment relates to the method of the first embodiment,wherein treating comprises a period of partial gas saturation to providea minimum crystallinity of 20% by weight and the gas concentration of 6%to 16% by weight.

A tenth embodiment relates to the method of the first through ninthembodiments, wherein the cellular poly(lactic acid) comprises cellshaving a size of 5 μm to 100 μm.

An eleventh embodiment relates to the method of the first through tenthembodiments, wherein the cellular poly(lactic acid) article has adensity that is less than or equal to 40% of the density of the solidpoly(lactic acid).

A twelfth embodiment relates to the method of the first through eleventhembodiments, wherein the cellular poly(lactic acid) article has acellular poly(lactic acid) structure within the interior and an integralnoncellular poly(lactic acid) layer at the surface.

A thirteenth embodiment relates to a method for making a shaped productfrom solid poly(lactic acid), including treating solid poly(lactic acid)that results in the solid poly(lactic acid) having a minimumcrystallinity of 20% by weight based on the weight of the solidpoly(lactic acid) and a gas concentration of 6% to 16% by weight basedon the weight of the solid poly(lactic acid); heating the solidpoly(lactic acid) having said minimum crystallinity and said range ofgas concentration to produce a cellular poly(lactic acid) article thatis shapeable; and shaping the cellular poly(lactic acid) into a product.

A fourteenth embodiment relates to the method of the thirteenthembodiment, wherein the cellular poly(lactic acid) article comprises agas concentration of essentially 0% by weight before shaping.

A fifteenth embodiment relates to the method of the thirteenth andfourteenth embodiments, wherein shaping comprises applying heat and atleast one of pressure or vacuum to the cellular poly(lactic acid).

A sixteenth embodiment relates to the method of the thirteenth throughfifteenth embodiments, wherein shaping comprises molding the cellularpoly(lactic acid) to the shape of a mold.

A seventeenth embodiment relates to the method of the thirteenth throughfifteenth embodiments, wherein the solid poly(lactic acid) is a rod orsheet.

An eighteenth embodiment relates to a poly(lactic acid) having cellsless than 100 microns (μm), a maximum density relative to the density ofsolid PLA of 40%, a percent tensile elongation before break of 10% to50%, and a minimum crystallinity of 20%.

A nineteenth embodiment relates to the poly(lactic acid) of eighteenthembodiment having the form of a sheet or rod.

A twentieth embodiment relates to the poly(lactic acid) of theeighteenth and nineteenth embodiments, wherein the poly(lactic acid) hasa cellular poly(lactic acid) structure within the interior and anoncellular poly(lactic acid) layer at the surface.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This summary is not intended to identify key features ofthe claimed subject matter, nor is it intended to be used as an aid indetermining the scope of the claimed subject matter.

DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same become betterunderstood by reference to the following detailed description, whentaken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a flow diagram of a related art method for foaming solidthermoplastic materials;

FIG. 2 is a flow diagram of a method for making a cellular poly(lacticacid) article that is shapeable;

FIG. 3 is a flow diagram of a method for making a shaped cellularpoly(lactic acid) article;

FIG. 4 is a diagrammatical illustration of equipment to produce a shapedcellular poly(lactic acid) article;

FIG. 5 is graph of the equilibrium carbon dioxide gas concentration insolid PLA as a function of saturation pressure at 20 C, wherein thePLA-CO₂ system exhibits behavior consistent with Henry's Law of sorptionand has a solubility of 42.93 mg/(g of PLA)/MPa;

FIG. 6 is graph of percent crystallinity by weight for CO₂ saturatedsolid PLA after saturation and prior to foaming as a function of the gasconcentration upon achieving equilibrium saturation;

FIG. 7 is a graph of bubble (cell or pore) size after foaming as afunction of equilibrium gas concentration after saturation formicrocellular PLA samples foamed at various foaming temperatures andalso showing the crystallinity that resulted after saturation and priorto foaming for these samples on the top axis;

FIG. 8 is graph of percent tensile elongation at break for solid-statemicrocellular PLA as a function of relative density;

FIG. 9 is a graph of relative density versus desorption time formicrocellular PLA samples processed at 3 MPa CO₂ saturation pressure andvarious desorption and foaming conditions, wherein each of the datapoints is an average density of two samples and the error bars showdifference in the densities of the two samples;

FIG. 10 is a graph of relative density versus desorption time for 4 MPasamples, wherein data points are the average of two sample measurementsand the error bars show difference in the sample values;

FIG. 11 is a graph of relative density versus desorption time for 5 MPasamples, wherein data points are the average of two sample measurementsand the error bars show difference in these sample values;

FIG. 12 is a graph of the relative density versus desorption time forall pressures (3, 4, and 5 MPa) grouped by desorption and foamingconditions with higher foaming and lower desorption temperaturesshifting the curves to the right;

FIG. 13 is a graph of gas concentration of samples desorbed at 0° C.just prior to foaming as a function of desorption time, wherein foamingtemperature is indicated for each trend only as a sample identifier andhas no effect on the data as the gas concentration was measured beforefoaming;

FIG. 14 is a plot of gas concentration of samples desorbed at −20° C.just prior to foaming as a function of desorption time, wherein foamingtemperature is indicated for each trend only as a sample identifier andhas no effect on the data as the gas concentration was measured beforefoaming (the low point on the graph is an outlier);

FIG. 15 is a plot of relative density versus gas concentration forsamples foamed at 80° C., showing that the relative density is notdependent on saturation pressure or desorption temperature;

FIG. 16 is a graph of relative density versus gas concentration forsamples foamed at 100° C., showing that the relative density is notdependent on saturation pressure or desorption temperature;

FIG. 17 is a graph of relative density versus gas concentration forsamples foamed at 80° C. and 100° C., showing that the relative densityis not dependent on saturation pressure or desorption temperature andfurther showing a shift for different foaming temperatures;

FIG. 18 is a graph of pore size as a function of saturation pressure formicrocellular PLA samples foamed in an infrared oven for various foamingtemperatures;

FIG. 19 is a graph of pore size as a function of foaming temperature formicrocellular PLA samples foamed in an infrared oven for varioussaturation pressures;

FIGS. 20A,B,C are cross sectional scanning electron micrographs ofrepresentative samples of cellular PLA;

FIG. 21 is a photograph of samples saturated at 5 MPa and heated to 40 Cin infrared;

FIG. 22 is a photograph of samples saturated at 5 MPa and heated to 60 Cin infrared;

FIG. 23 is a photograph of samples saturated at 5 MPa and heated to 80 Cin infrared;

FIG. 24 is a photograph of samples saturated at 5 MPa and heated to 100C in infrared;

FIG. 25 is a photograph of samples saturated at 4 MPa and heated to 40 Cin infrared;

FIG. 26 is a photograph of samples saturated at 4 MPa and heated to 60 Cin infrared;

FIG. 27 is a photograph of samples saturated at 4 MPa and heated to 80 Cin infrared;

FIG. 28 is a photograph of samples saturated at 4 MPa and heated to 100C in infrared;

FIG. 29 is a photograph of samples saturated at 3 MPa and heated to 40 Cin infrared;

FIG. 30 is a photograph of samples saturated at 3 MPa and heated to 60 Cin infrared;

FIG. 31 is a photograph of samples saturated at 3 MPa and heated to 80 Cin infrared;

FIG. 32 is a photograph of samples saturated at 3 MPa and heated to 100C in infrared;

FIG. 33 is a photograph of samples saturated at 2 MPa and heated to 40 Cin infrared;

FIG. 34 is a photograph of samples saturated at 2 MPa and heated to 60 Cin infrared;

FIG. 35 is a photograph of samples saturated at 2 MPa and heated to 80 Cin infrared;

FIG. 36 is a photograph of sample 1 (FIG. 33, left) saturated at 2 MPaand heated to 100 C in infrared; and

FIG. 37 is a photograph of sample 2 (FIG. 33, right) saturated at 2 MPaand heated to 100 C in infrared.

DETAILED DESCRIPTION

Poly(lactic acid) (herein referred to as “PLA”) is a biodegradable,thermoplastic, high modulus polymer that can be obtained from annuallyrenewable resources, such as corn starch or sugar cane. In addition tothe wide applications in biomedical fields, recently developed andcommercially available solid PLA has made it feasible for use as apackaging material. It may be used as a renewable degradable plastic foruses in service ware, mulch films, grocery waste, and composting bags,etc. In order to be made into such products, solid PLA is shaped by heatand pressure, for example, such as in thermoforming. To save onmaterial, a cellular shapeable PLA article is disclosed herein that canbe shaped by heat and pressure into useful articles similar to solidPLA. The cellular PLA articles disclosed herein that are shapeableinclude desirable properties, such as a smooth and glossy appearance,are ductile to allow for shaping, and are rigid once shaped. A methodfor making the cellular PLA articles that are shapeable are alsodisclosed, as well as a method for shaping the cellular PLA articlesinto articles.

PLA is a semicrystalline amorphous solid. In this application,crystallinity is given as a percent by weight based on the total weightof the PLA. Crystallinity percent by weight is measured using adifferential scanning calorimetry (DSC) technique. A suitable instrumentfor measuring crystallinity percent includes the Nitzche DSC instrumentwith a computerized data acquisition system. The enthalpies of fusionand crystallinity of PLA were determined according to the standard ASTMD3417-97. Generally, the density of PLA is in the range of 1.21 g/cm³ to1.43 g/cm³. A preferred density of a cellular PLA article disclosedherein would be about 40% (40% relative density) of the original densityof the solid PLA or less.

The inventors' early attempts at foaming PLA via a solid state methodoften resulted in the foamed article being warped, corrugated, and/orwrinkled due to violent foaming because of the greater quantities of gasbeing released from PLA very rapidly. The foamed article wasgeometrically and mechanically unsuitable for thermoforming or moldingapplications and often had either large cell sizes or blisters. When PLAis subjected to the solid-state microcellular foaming process of FIG. 1,it was found that the rate at which PLA absorbs and desorbs carbondioxide gas is significantly higher compared to thermoplastic materialslike poly(ethylene terephthalate) andpoly(acrylonitrile-butadiene-styrene). The high diffusion rate of PLAcreates processing issues during desorption and foaming.

To successfully make cellular PLA articles that are shapeable, it wasdiscovered that immediately before foaming via the application of heat,the solid PLA included a gas concentration in the range of 6% to 16% byweight based on the weight of PLA and included a minimum crystallinityof 20% by weight based on the weight of PLA. Foaming solid PLA withthese characteristics results in a cellular PLA article having theproperties that render the article suitable for shaping via heat andpressure (or vacuum). The cellular PLA article has a maximum relativedensity of 40% or less compared to the density of the noncellular PLA.The cellular PLA article has a minimum crystallinity of 20% by weightbased on the weight of the PLA. The cellular PLA article has a smoothglossy outer integral PLA skin that is noncellular and a cellular PLAstructure in the interior. The cellular PLA article has cells less than100 microns, generally in the range of size of 5 to 100 μm. The cellularPLA article has a percent tensile elongation before break in the rangeof 10% to 50%, and the cellular PLA article remains rigid once shaped byheat and pressure (or vacuum).

Referring to FIG. 2, one embodiment of a method for producing a cellularpoly(lactic acid) article that is shapeable is provided. Shapeablerefers to the ability of the article to be transformed into a differentform or shape through the application of heat and pressure (or vacuum),such as through thermoforming. The method illustrated in FIG. 2 includesblock 200. In block 200, PLA is obtained. The PLA obtained can beprovided as a rod or rolled sheet of PLA material. A representativesupplier of PLA is Ex-Tech Plastics of Richmond, Ill., U.S.A., which canprovide PLA as a sheet having a thickness of 0.60 mm. The resin fromwhich the sheet was extruded is PLA 2002D (FG Grade) made byNatureWorks™ LLC of Minnetonka, Minn. This PLA material has a density of1.24 g/cm³ and a glass transition temperature of 55° C. Thecrystallinity percent of the as-received solid PLA material is below 5%,and the percent tensile elongation before break is 6%.

From block 200, the method enters block 202. In block 202, the solid PLAis treated under pressure with one or more gases. The PLA can beenclosed in a sealed pressure vessel, for example, from which air hasbeen evacuated or purged and then any one or more suitable gas or gasesis introduced into the pressure vessel. Representative of the one ormore gases are carbon dioxide and/or nitrogen. A suitable pressure atwhich to treat the solid PLA in step 202 is from 2.75 MPa to 7 MPa.Preferably, the range is from 3 MPa to 5 MPa. In block 202, the solidPLA can be saturated completely followed by block 204. In block 204, thesolid PLA treated in block 202 is treated at a lower pressure and/or ata lower temperature to allow the one or more gases to desorb to achievepartial saturation. Desorption can proceed at ambient atmosphericpressure and at a temperature of −20° C. to 25° C. Desorption proceedsuntil the concentration of the one or more gases in the PLA is 6% to 16%by weight based on the weight of the PLA. In an alternative embodiment,in block 202 the one or more gases only partially saturates into thesolid PLA. The concentration of the one or more gases in the latterembodiment is also in the range of 6% to 16% by weight based on theweight of the PLA. Block 204, which provides for desorption of gas,allows for the creation of cellular PLA articles that have an integralnoncellular outer skin. Integral refers to being formed from the samePLA material that also forms the cellular structure of the interior.Furthermore, it was found that in order to make many small cells thatresults in the desired density reduction, the crystallinity is preferredto be a minimum of 20% by weight based on the weight of the PLA, for therange of saturation pressures of 3 MPa to 5 MPa and gas concentration inthe range of 6%-16%. A crystallinity minimum of 20% by weight produces amajority of cells in the range of 5 μm to 100 μm. The highercrystallinity also leads to a greater number of nucleation sites forcell growth. When PLA is heated having a crystallinity of a minimum 20%by weight and a gas concentration range of 6% to 16% by weight, thenumber and size of the cells that result provide the cellular PLA with arelative density compared to the virgin material of 40% or less. Thecellular PLA also has the proper outer appearance, is generally flat, isductile and rigid once shaped by heat and pressure (or vacuum) Processconditions of blocks 202 and/or 204 are adjusted so that the solid PLAachieves a crystallinity of at least 20% by weight based on the weightof the solid PLA and also achieves a gas concentration in the range of6% to 16% by weight based on the weight of the solid PLA immediatelybefore foaming, block 206. From block 206, the method enters block 208.

In block 208, the solid PLA having a crystallinity by weight of at least20% and a gas concentration of 6% to 16% by weight is heated to createthe cellular structure. In one particular embodiment, the temperaturefor foaming of block 208 is in the range of 40° C. to 100° C. Heating,particularly in the case for commercial application can be by aflotation/impingement air oven that can accept sheets and rods of PLA.The result of heating the solid PLA at the specified crystallinity andgas concentration ranges results in a shapeable cellular PLA article.Shapeable as used herein means the ability to be formed to a new shapeby the application of heat, pressure (or vacuum), or both. The shapeablecellular PLA article has cell sizes less than 100 microns, a maximumdensity relative to the density of solid PLA of 40%, a percent tensileelongation before break of 10% to 50%, and a minimum crystallinity of20%. The cell size is generally in the range of 5 μm to 100 μm. Theshapeable cellular PLA article has a cellular PLA structure within theinterior and an integral non-cellular PLA layer on the surface. Theintegral non-cellular layer on the exterior is produced by providing astep for gas desorption, block 204. The shapeable PLA article can havethe form of a rod, sheet or roll.

Referring to FIG. 3, another embodiment of a method in accordance withthe present disclosure is illustrated. Blocks 300-310 are similar toblocks 200-210, respectively of the embodiment illustrated in FIG. 2.The method represented by FIG. 3 includes block 312. Block 312 is forshaping the shapeable cellular PLA article produced in block 310.Shaping can be done by any process that uses heat and/or pressure (orvacuum) to change the shape of the cellular PLA article, such as bythermoforming or molding. The shapeable cellular PLA article of block310 is allowed to undergo desorption of gas so that the gasconcentration is essentially 0% by weight before the shaping process,block 312. A shaped cellular PLA article is produced, block 314. Theshaped cellular PLA article can be packaging, for example.

FIG. 4 is a diagrammatical illustration of equipment for making 1) thecellular PLA article that is shapeable, and 2) a shaped article fromcellular PLA. Rolls of PLA sheet 404 are placed in a pressure vessel400. The pressure vessel 400 may be sealed and evacuated and/or purgedof any remaining air. Carbon dioxide is introduced into the pressurevessel 400. The temperature (T_(absorb)) of the pressure vessel 400 ispreferably ambient room temperature. However, a higher or lowertemperature may be used. The pressure (P_(absorb)) of the pressurevessel 400 can be in the range of 2.75-7 MPa, and more particularly inthe range of 3-5 MPa. In order to have carbon dioxide fully saturate theinner rolls of the sheet, a porous material, such as paper, can beinterleaved between each roll of the sheet. Alternatively, individualflat sheets can be interleaved with the porous material and saturatedwith gas in a stacked arrangement. A roll is being used merely as arepresentative material and is not intended to be limiting. The amountof time for complete saturation can be determined beforehand. Forexample, a test using a sample roll of PLA can be conducted at the sametemperature and pressure conditions and at various time intervals. Thesample can be pulled from the pressure vessel and measured for weight.When the weight of the sample ceases to increase over time, the samplehas reached complete saturation for the given temperature and pressure.The time can be noted, and various tables for complete saturation can becreated for any given combination of temperature and pressureconditions. Knowing those conditions, the time that the PLA rolls 404remain within the pressure vessel can be known. After completesaturation, the PLA rolls 404 can be moved to a cold environment (suchas a refrigerator, freezer, etc.) 402. The PLA rolls 404 can be treatedat a different temperature (T_(desorb)). The temperature for desorptioncan be, for example, −20° C. to 25° C. The pressure (P_(desorb)) can bea pressure lower than P_(absorb) and may even be a vacuum. The vessel402 can be the same or a different vessel than 400. After treatment invessel 402, the PLA rolls have a minimum crystallinity of 20% by weightand a gas concentration of 6% to 16% by weight. Similar to the testingfor the complete saturation of gas, a test can be conducted beforehand,where a completely saturated test roll of PLA is placed in vessel 402,and the test roll is sampled at specified intervals at the specifiedconditions in order to determine the amount of time necessary for thecompletely saturated sample roll of PLA to reached the desiredcrystallinity and gas concentration at the specified temperature andpressure. Knowing the time of desorption for any given combination oftemperature and pressure, the amount of time to reach the desiredconditions in vessel 402 is known. A PLA roll 404 having the desiredcrystallinity and gas concentration is placed in a holder, such asholder 405. The solid state foaming equipment is indicated by numerals408 and 410. This equipment can be a flotation/impingement air oven orinfrared oven. The equipment includes an upper section 408 and a lowersection 410, wherein the PLA roll 404 is unwound and passed into theflotation/impingement air oven to receive heat from both the upper andlower sections of the oven. At this point, the porous, interleavedmaterial can be discarded. The solid PLA sheet having the desiredcrystallinity gas and concentration ranges 406 is fed through theflotation/impingement air oven 408, 410. As it traverses the oven, thesolid sheet 406 is converted into a cellular PLA sheet 418 havingcharacteristics suitable for thermoforming. The sheet can ride onconveyor rollers 412 and 414 before and after the flotation/impingementair oven to make the process a continuous process. The oven can heat thePLA sheet 418 to a temperature in the range of 40° C. to 100° C., for aduration of between 5 to 300 seconds. The process in FIG. 4 isillustrated as a continuous process, wherein the shaping of cellular PLAis performed following the making of the cellular PLA. However, in otherembodiments, the process may include an additional resting period, suchas block 428, following the foaming of the cellular PLA to allow thecellular PLA to desorb of essentially all gas before the thermoformingprocess. Thermoforming is merely described as one representative exampleof a shaping process and is not intended to be limiting.

The equipment labeled 420 and 422 is representative of a thermoformingmachine. The thermoforming machine includes an upper section 420 and alower section 422. Within the thermoforming machine, a mold or cavity426 is provided in the shape of the desired thermoformed article. Thelower section 422 may be stationary and the upper section 420 may bedriven by a press with a negative of the mold 426. Thermoforming machine420 and 422 can include heating elements (not shown). The heatingelements can heat the cellular PLA sheet 418 to a temperature in therange of 160-240° F. before passing into the mold 426. The mold 426 maybe heated in the range of 70-150° F. In addition to the press 420 and/orin lieu of the press 420, the lower section of the thermoforming machine422 can include ducts which apply vacuum to the mold cavity 426 and thusassist in the shaping process. The cellular PLA sheet 418 is carried onconveyor roller 416 to enter the thermoforming machine in a continuousor semicontinuous process from the floatation/impingement air oven. Inanother alternative embodiment, after the creation of the cellular PLAsheet 418, the sheet 418 can be allowed to desorb of gas to essentiallyreach 0% by weight of gas concentration before being thermoformed.Shaped cellular PLA articles 424 are produced, such as trays, packaging,etc.

Referring to FIG. 17, a graph of relative density plotted against gasconcentration is illustrated. The data shows that irrespective ofdesorption temperature and saturation pressure, at a given foamingtemperature a sample of a certain gas concentration can be expected tohave a certain relative density. This is useful as it providesflexibility in process parameters to achieve a desired density goal. Itshould be noted, however, that this result will not necessarily hold forlower saturation pressures. Density can be measured according to ASTMstandard D792. The flotation weight loss method uses distilled water asthe liquid. The sample is first weighed “dry,” and then the sample isplaced below the surface of the water and weighed again. The equationused to calculate the density of the polymer sample is:

$\begin{matrix}{D = {\left( \frac{W_{d}}{W_{d} - W_{w}} \right) \cdot D_{w}}} & (1)\end{matrix}$

where,

D=density of the sample

W_(d)=dry weight

W_(w)=wet weight

D_(w)=density of distilled water (taken as 0.9975 g/cm³)

Density is reported as relative density or void fraction. Relativedensity is the density of the foamed material divided by the density ofthe unfoamed material. Void fraction is defined as one minus therelative density. Both relative density and void fraction are expressedas a percentage. For example, a material with 60% relative density meansthat the total volume of the foamed sample is 60% polymer and 40% air.At a given relative density this would have a large effect on bubblesize, appearance, feel and shapeability of the cellular PLA, as thecrystallinity in the material at a given gas concentration changes theway the material expands. At approximately below 20% crystallinity,large bubbles (above 100 μm) are formed in the cellular PLA. This is dueto two reasons: (1) fewer crystallites exist in the material leading tofewer nucleation sites for bubbles; and (2) the crystalline matrix doesnot prevent the continued bubble growth in the amorphous regions of thepolymer. These large bubbles lead to a coarse looking foam sheet (orarticle) that is flexible, mostly translucent, and incapable of making arigid shaped article upon thermoforming.

However, at or above 20% crystallinity bubbles are in the microcellularrange (below 100 μm). This is because (1) there are significantly morecrystallites and hence nucleations sites available for bubble formation;and (2) the crystallite network arrests bubble growth in the amorphousregions of the polymer by reducing molecular mobility. Thesemicrocellular bubbles lead to a fine looking cellular PLA sheet withintegral skin that is rigid, mostly opaque, and capable of being shapedinto a rigid shaped article upon thermoforming.

The relative density versus gas concentration plots also indicate thatthe reason for the vastly different and unsuitable morphology of the 5MPa samples at low desorption times may be only due to the high gasconcentration in the sample. In other words, at approximately 16% byweight of gas (carbon dioxide) concentration (and higher), thediffusivity of CO₂ in PLA is so high that the expanding PLA sheetundergoes an explosive collapse in bubble structure that results inrelative densities higher than 40% and a collapsed bubble structure. Atapproximately 6% gas concentration and below, there is not enough gas tocreate small cells to achieve a rigid, integrally skinned, and shapeablecellular PLA sheet (generally, the lower the gas concentration thelarger the cells and vice versa). Hence, the gas concentration forcreating shapeable PLA sheets is below 16% by weight of carbon dioxidegas concentration, and above 6% by weight in order to achieve at least arelative density of 40% along with cells below 100 μm compared to thenoncellular PLA.

FIG. 6 is graph of percent crystallinity by weight for CO₂ saturatedsolid PLA after saturation and prior to foaming as a function of the gasconcentration upon achieving equilibrium saturation.

FIG. 7 is a graph of bubble (cell or pore) size after foaming as afunction of equilibrium gas concentration after saturation formicrocellular PLA samples foamed at various foaming temperatures andalso showing the crystallinity that resulted after saturation and priorto foaming for these samples on the top axis. The characterization ofmicrocellular structures can be performed by imaging the structures witha scanning electron microscope (SEM), such as a digital FEI Sirironscanning electron microscope. Samples can be scored and freeze fracturedwith liquid nitrogen. Samples can then be mounted in metal stages andthe imaged surface sputter coated with Au—Pd for between 20 to 60seconds. When reporting the size of cells, the measurement is of thecell's largest dimension. FIGS. 20A, B, and C show representativesamples of cellular PLA, showing the internal cellular PLA structure2002 and the integral and noncellular PLA layer 2000.

FIG. 8 is graph of percent tensile elongation at break for solid-statemicrocellular PLA as a function of relative density. Tensile testinggenerally requires the application of a gradually increasing uniaxialstress until the propagation of a single crack causes failure. Samplesfor tensile testing can be manufactured according to ASTM D638 Type IVspecifications. Testing of these samples can follow ASTM D638. Tensiletesting can be performed on any suitable equipment, such as an Instron5585H. In this apparatus, serrated jaws hold the tensile samples. Aconstant crosshead rate is used to control the amount of stress appliedto the polymer samples.

EXAMPLE

The following example demonstrates the creation of a cellular PLAarticle that is shapeable.

1. Material

Extruded PLA sheet, thickness 0.60 mm (=0.024 inch), made by Ex-TechPlastics was procured for this example. The resin from which the sheetwas extruded is PLA 2002D (FG grade) and was made by NatureWorks™ LLC.In the as received condition, Ex-Tech reported that the material has adensity of 1.24 g/cm³ (=1240 kg/m³=77.4 lb/ft³) and a T_(g) of 55° C.(=131 F). The PLA sheet was prepared into 38.1 mm×38.1 mm (1.5 inch×1.5inch) square samples. The samples were used in these experiments in theas-received condition.

2. Experimental

2.1 Equipment

For the gas saturation step, a 101.6 mm (=4 inch) diameter and 101.6 mm(=4 inch) deep carbon steel pressure vessel was used. The pressurevessel is rated for use up to a maximum pressure of 10.34 MPa (=1500psi) at 0 C (=32 F). The pressure inside the vessel was regulated usingan OMEGA CN8500 process controller with a resolution of ±0.01 MPa (=1.45psi). A Mettler-Toledo AE240 precision balance with an accuracy of 10 μg(=2.2E-9 lb) was used to measure the gas solubility and relativedensity. The foaming was carried out in a heated circulating water bathcontrolled by a Techne TE-10D heater/circulator with a workingtemperature range of ambient +5 to 120° C. and set point accuracy of ±1°C. The samples were desorbed in a temperature controlled freezer made byFreezer Concepts Inc., model CT30-2. The PID controller for the freezeris by FutureDesign Controls with an accuracy of +/−2 C at 25 C.

2.2 Procedure

Solid PLA samples (38.1 mm×38.1 mm) were saturated to equilibriumconcentration at the various saturation pressures (3, 4 and 5 MPa) withCO₂ gas. The samples were removed two at a time (with the pressurevessel re-sealed and re-pressurized after each removal). The mass of thetwo samples was measured immediately after depressurization and then thesamples were foamed for 2 minutes in the circulating water bath. Themass of the samples prior to saturation was recorded for calculation ofCO₂ concentration prior to and after foaming. Once the samples weresaturated, they were removed from the pressure vessel, their weightswere recorded, and then placed in a plastic bag in a freezer that wasset to the appropriate temperature by a PID controller. The samples werethen removed at regular intervals, the mass recorded again (aftercondensation had evaporated from the sample surface), and then foamed atthe appropriate temperature for two minutes in the temperaturecontrolled, circulating water bath. After foaming the density of each ofthe samples was measured according to ASTM standard D792-91 and otherobservations of the quality of the foam were noted. The samples' densitywas measured after a significant amount of time had passed such that theresidual gas concentration in the expanded microcellular PLA samples wasnegligible. Table 1 lists the set numbers and processing conditions. Inthis example, three saturation pressures of 3, 4 and 5 MPa, twodesorption temperatures of 0 C and −20 C and two foaming temperatures of80 C and 100 C are described.

TABLE 1 SAMPLE SETS AND PROCESSING CONDITIONS Sample SaturationDesorption Foaming Set ID Pressure (MPa) Temp. (C.) Temp (C.) D 3 0 80 E3 −20 80 F 4 0 80 G 4 −20 80 H 4 0 100 I 4 −20 100 J 3 0 100 K 3 −20 100L 5 0 100 M 5 −20 100 N 5 0 80 O 5 −20 80

2.3 Observations

A large number of samples (a total of 230) were prepared for andprocessed during this set of experiments in order to get a detailed viewof the combined effects of saturation pressure, desorption time,desorption temperature, and foaming temperature.

Several effects were observed in a few of the sample groups. Duringfoaming a few of the samples blistered from the inside and theseinternal blisters had sizes between 2 mm and 20 mm. These blisters wereseen intermittently in all groups of samples except for the sets listedin Table 2. A clear trend for these large internal blisters was notapparent when scrutinizing all of the samples. As a general trend theinternal blisters appeared with shorter desorption time and higher gasconcentration.

Another effect that was observed was the appearance of small surfaceblisters that cause surface roughness. At 5 MPa, the surface blisterswere very apparent and uniformly distributed across the surface of thesamples that had short desorption times. This effect is a result ofviolent out-gassing of CO₂ (due to high diffusivity of CO₂ in PLA athigh gas concentrations) during the foaming step. This effect is drivenby the high gas concentration in the 5 MPa samples of short desorptiontimes, which is much higher than in other samples. A similar phenomenonof surface roughness is seen at lower pressures (below 2.75 MPa) but isa much more subtle effect caused by the thin solid skin, which resultsin samples that are not desorbed for long, and thus the bubbles thatform under the surface deform the skin and give it a rough appearance.

The oddest result that was observed among all the expanded PLA sampleswas the non-uniformity of foaming in most of the 5 MPa saturatedsamples. After foaming there appeared an off-centered region (about 25%of the total sample area) that was of much higher density than the restof the sample. The cause of this irregularity is unclear.

TABLE 2 SAMPLE SETS IN WHICH BLISTERS WERE NOT OBSERVED SampleSaturation Desorption Foaming Set Pressure (MPa) Temp. (C.) Temp (C.) D3 0 80 E 3 −20 80 F 4 0 80 G 4 −20 80 N 5 0 80

3. Results and Discussion

Along with visual observation on aesthetics, for each of the samplesets, the relative density was measured. For each sample set, therelative density was first correlated with desorption time andsaturation pressure. This data is displayed in FIGS. 9-11. Desorptiontime is given on a log scale to better depict the effects at shortdesorption times. All three graphs shown in FIGS. 9-11 display the sametrend in their data; the curves are shifted to the right with increasingfoaming temperature and decreasing desorption temperature. This meansthat an increase in foaming temperature reduces the relative density anda lower desorption temperature slows gas desorption out of the sampleresulting in a lower relative density when foamed.

To better visualize these trends, all the data was plotted on the samegraph as shown in FIG. 12. It is clear that desorption and foamingtemperatures have a more dominant effect on relative density at highdesorption times than saturation pressure. The plots are clustered bythe paired parameters of foaming temperature and desorption temperaturewith samples desorbed at 0 C and foamed at 80 C having higher relativedensity and samples desorbed at −20 C and foamed at 100 C having lowerrelative density. Close inspection of FIGS. 9-11 shows that the mostdominant parameter is desorption temperature. For samples processed with−20 C desorption temperature a processing window exists in which acertain low relative density may be attained. For a relative density ofaround 10%, this desorption time window is 3, 6, and 8 hours for 3, 4,and 5 MPa saturation pressures respectively.

For each desorption temperature, the gas concentration versus desorptiontime data was also plotted (FIGS. 13 and 14). The gas concentration wasobtained by recording the mass of the samples prior to saturation andprior to foaming. Thus, this gas concentration calculation is of the gassaturated and desorbed PLA sheet just before foaming. The data isidentified by the temperature that the samples were to be foamed at, butthis is mostly irrelevant as the gas concentration is from pre-foamedsamples. The graphs show that gas desorbs more slowly at −20 C, and bothgraphs seem to be converging to zero gas concentration. This means thatafter a certain period of desorption, a sample from a certain saturationpressure cannot be distinguished from another sample from a differentsaturation pressure. The smaller slope of the −20 C desorption is thereason that these samples show a lower density for a longer period oftime.

These initial plots lead to the conclusion that the most influentialparameter on relative density is the gas concentration. Thus, relativedensity was plotted against gas concentration for each foamingtemperature (FIGS. 15 and 16). All the data (for both foamingtemperatures) are plotted one on top of the other in FIG. 17. This datashows that irrespective of desorption temperature and saturationpressure, a sample with a certain gas concentration can be expected tohave a certain relative density. This is very useful as it providesflexibility in process parameters to achieve a desired density goal. Itshould be noted, however, that this result will not necessarily hold forlower saturation pressures.

Crystallinity is rather uniform between 3 and 5 MPa, but is much lowerfor lower pressures (below 2.75 MPa). This would have a large effect onthe bubble size, appearance, rigidity, and shapeability as thecrystallinity changes the way the material expands. The relative densityversus gas concentration plots also indicate that the reason for thevastly different morphology of the 5 MPa samples at low desorption timesmay be only due to the high gas concentration in the sample. In otherwords, at approximately 16% by weight of CO₂ concentration (and higher),the expanding PLA sheet undergoes a collapse in bubble structure thatresults in higher relative densities. Hence, the optimal gasconcentration for creating thermo formable PLA sheets is below 16% byweight of CO₂ gas concentration.

4. Conclusions

For a given foaming temperature, gas concentration and crystallinityprior to foaming are the main variables influencing the relativedensity, bubble size, rigidity, appearance, and shapeability of cellularPLA.

Processing ranges for gas concentration/crystallinity at a givendesorption temperature/desorption time pair have been determined inwhich a consistent density, bubble size, rigidity can be reached.

Lower desorption temperature significantly increases the processingwindow for PLA.

Thermoforming quality, solid-state, expanded, microcellular PLA can becreated using CO₂ saturation pressures in the range of 3-5 MPa at roomtemperature, CO₂ gas concentrations between 6-16% by weight, desorptiontemperatures in the range of −20 C to 25 C and foaming temperatures inthe range of 40-100 C.

REPRESENTATIVE THERMOFORMING EXAMPLE 1. Material

Extruded PLA sheet, thickness 0.60 mm (=0.024 inch), made by Ex-TechPlastics was procured for this example. The resin from which the sheetwas extruded is PLA 2002D (FG grade) and was made by NatureWorks™ LLC.In the as received condition, Ex-Tech reported that the material has adensity of 1.24 g/cm³ (=1240 kg/m³=77.4 lb/ft³) and a T_(g) of 55° C.(=131 F). The PLA sheet was prepared into 10 inch×10 inch squaresamples. The samples were used in these experiments in the as-receivedcondition.

2. Equipment

For the gas saturation step, a 585 mm diameter and 1.8 m deep carbonsteel pressure vessel was used. The pressure vessel is rated for use upto a maximum pressure of 9 MPa at 65 C. The pressure inside the vesselwas regulated using a pressure gauge with a resolution of 0.7 MPa and anaccuracy of ±5% over the range. For CO₂ saturation study the gas supplysystem to the pressure vessel can deliver a maximum pressure of 5 MPa at22 C.

For the expansion step heating of the gas saturated samples was carriedout between two 760 mm square IR panels (top and bottom). The IR panelshave an inner and outer heating zone. In order to keep the foaming PLAsamples relatively flat the foaming in between the IR panels wasconducted using a spring frame that allows the samples to expandin-plane while foaming without getting too close to either of the upperor lower IR panels. A precision balance with an accuracy of 100 μg wasused to measure the density of the foamed HIPS samples.

For the thermoforming step, a Illig Prototyping Sheet Thermoformer witha 127 mm×178 mm×25 mm rectangular food packaging tray mold was used. Thethermoforming machine used relay-based software control when functioningin the automatic mode. The thermoforming machine was capable of pressureand vacuum thermoforming along with a third motion plug assist that wasdriven independent of the machine platens by an air cylinder. Thethermoforming machine had top and bottom ceramic heaters with 16 zonecontrol. The available forming area in the thermoformer was 8 inch×8inch. The distance between the heater surface and the sheet line in thethermoformer was 3 inch.

3. Procedure

Multiple samples of 6 inch×6 inch solid PLA were interleaved a porousmaterial and saturated in the pressure vessel with CO₂ gas at roomtemperature at various saturation pressures of 2, 3, 4 and 5 MParespectively. Upon saturation at a given pressure the samples wereremoved from the pressure vessel and foamed immediately in the IR ovenby loading into the spring frame at temperatures of 40 C, 60 C, 80 C and100 C. The target relative densities were below 40%. Each IR heater hasan inner and outer heat zone. The inner zone was set to a temperature of343 C and the outer zone (7.5 cm wide along the perimeter) was set to399 C.

After foaming the samples were allowed to desorb gas for approximately 1week before thermoforming such that the CO₂ gas concentration in thefoamed material was essentially zero. For the thermoforming experimentsthe thermoforming machine's IR heaters were set to a temperature of350-450 F. Each of the foamed sheets was heated for approximately 7seconds such that the surface temperature of the material wasapproximately 190 F. The thermoforming mold whose temperature wascontrolled by heated/cooled oil circulation was set to a temperature of106 F. The thermoforming plug whose temperature was controlled byheated/cooled water was set to a temperature of 75 F. The forming airpressure was set to 40 psi and the forming time used was 5 seconds.

4. Results and Discussion Samples Saturated at 5 MPa

Samples saturated at 5 MPa did not show good expansion. The samplesheated to 40 C did not foam at all and the rest of the foamingtemperatures did not give foam that was much larger than the originalvirgin sheet. See FIGS. 21 through 24. Samples heated to 80 and 100 Cdisplayed small blisters on the surface and relative density that washigher than 40%. The gas concentration in the material just prior tofoaming was approximately 21% by weight and crystallinity was above 20%.The material was too brittle to load in the thermoformer and would keepcracking when loading was attempted.

Samples Saturated at 4 MPa

Samples saturated at 4 MPa showed more expansion than the 5 MPa samples,particularly for the 100 C samples. The gas concentration in thematerial just prior to foaming was approximately 17% by weight andcrystallinity was above 20%. There was no evidence of small surfaceblisters. However, the foamed specimens were rather corrugated. Thesecellular PLA sheets could be loaded in the thermoformer but the foodpackaging trays had uneven appearance upon thermoforming due to thecorrugations in the material prior to thermoforming. (See FIGS. 25-28.)

Samples Saturated at 3 MPa

The foamed samples saturated at 3 MPa had the best appearance comparedto those saturated at 4 and 5 MPa. One of the 40 C samples was left inthe oven for too long (see FIG. 29, sample 1, on the left) and itfoamed. The samples foamed to 100 C expanded to about twice the originalarea of the sheet. The gas concentration in the material just prior tofoaming was approximately 12% by weight and crystallinity was above 20%.There was minimal corrugation in the foamed samples and no surfaceblisters. See FIGS. 29-32. These cellular PLA sheets could be loaded inthe thermoformer and the food packaging trays had a clean, evenappearance. The trays were rigid with a smooth surface and a milky whitefinish resulting from the integral skin and microbubbles.

Samples Saturated at 2 MPa

The 2 MPa samples did not foam at 40 C and hardly foamed at 60 C. At 80and 100 C, the samples foamed and stretched a lot, but the foam sampleswere translucent, uneven, and had large cells similar to the samplesfoamed in the heated water bath. The gas concentration in the materialjust prior to foaming was approximately 8% by weight and crystallinitywas below 20%. See FIGS. 33-37. These cellular PLA sheets could beloaded in the thermoformer. However, they were too flexible and resultedin food packaging trays that were floppy and non-rigid. The trays wereincapable of holding their shape when filled with water. They would bendand buckle severely under the load. The trays had a rough surface withlarge bubbles in the interior that gave them a translucent look and adull grey finish. The trays were capable of holding their shape whenfilled with water and there was no evidence of bending or buckling underthe load.

While illustrative embodiments have been illustrated and described, itwill be appreciated that various changes can be made therein withoutdeparting from the spirit and scope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Poly(lactic acid) havingcells less than 100 microns, a maximum density relative to the densityof solid poly(lactic acid) of 40%, a percent tensile elongation beforebreak of 10% to 50%, and a minimum crystallinity of 20%.
 2. Thepoly(lactic acid) of claim 1, having the form of a sheet or rod.
 3. Thepoly(lactic acid) of claim 1, wherein the poly(lactic acid) has acellular poly(lactic acid) structure within the interior and anoncellular poly(lactic acid) layer at the surface.
 4. The poly(lacticacid) of claim 1, wherein the density is greater than 5% to 40% relativeto the density of solid poly(lactic acid).
 5. The poly(lactic acid) ofclaim 1, which has been treated with heat while being shaped.
 6. Thepoly(lactic acid) of claim 5, which has a milky white color.